Because of their excellent magnetic properties, rare earth permanent magnets are frequently used in a wide variety of applications such as electric apparatus and computer peripheral devices and are important electric and electronic materials. In particular, a family of Nd—Fe—B permanent magnets has lower starting material costs than Sm—Co permanent magnets because the key element neodymium exists in more plenty than samarium and the content of cobalt is low. This family of magnets also has much better magnetic properties than Sm—Co permanent magnets, making them excellent as permanent magnets. For this reason, the demand for Nd—Fe—B permanent magnets is recently increasing and the application thereof is spreading.
However, the Nd—Fe—B permanent magnets have the drawback that they are readily oxidized in humid air within a short time since they contain rare earth elements and iron as the main components. When Nd—Fe—B permanent magnets are incorporated in magnetic circuits, the oxidation phenomenon raises such problems as decreased outputs of magnetic circuits and contamination of the associated equipment with rust.
In the last decade, Nd—Fe—B permanent magnets find incipient use in motors such as automotive motors and elevator motors. The magnets are inevitably used in a hot humid environment. In some potential situations, the magnets are exposed to salt-containing moist air. It would be desirable if magnets are endowed with higher corrosion resistance at low cost. In the manufacturing process of motors, the magnets can be heated at 300° C. or higher, though for a short time. In this application, the magnets are also required to have heat resistance.
To improve the corrosion resistance of Nd—Fe—B permanent magnets, various surface treatments such as resin coating, aluminum ion plating and nickel plating are often implemented. It is difficult for these surface treatments of the state-of-the-art to accommodate the above-mentioned rigorous conditions. For example, resin coating provides insufficient corrosion resistance and lacks heat resistance. Nickel plating allows the underlying material to rust in salt-containing moist air because of the presence of some pinholes. The ion plating technique achieves generally satisfactory heat resistance and corrosion resistance, but needs a large size apparatus and is thus difficult to conduct at low cost.